Which of the following statements is not correct ?

Options

A. Proteins are polyamides formed from amino acids.
B. Except glycine, all other amino acids show optical activity.
C. Natural proteins are commonly made up of L-isomer of amino acids.
D. In `alpha`-amino acids, `-NH_(2)` and `-COOH` groups are attached to different carbon atoms.

Explanation

To determine which statement is not correct, we will analyze each of the provided statements step by step.

### Step 1: Analyze Statement 1
**Statement:** Proteins are polyamides formed from amino acids.
**Analysis:** This statement is correct. Proteins are indeed polymers made from amino acids linked together by peptide bonds, which are a type of amide bond. Therefore, this statement is true.

### Step 2: Analyze Statement 2
**Statement:** Except glycine, all other amino acids show optical activity.
**Analysis:** Glycine is the only amino acid that is not optically active because it does not have a chiral center (it has two hydrogen atoms attached to its alpha carbon). All other amino acids have at least one chiral center and thus show optical activity. Therefore, this statement is also correct.

### Step 3: Analyze Statement 3
**Statement:** Natural proteins are commonly made up of L-isomers of amino acids.
**Analysis:** This statement is correct. In nature, amino acids in proteins are predominantly found in the L-form (levorotatory). Therefore, this statement is true.

### Step 4: Analyze Statement 4
**Statement:** In alpha amino acids, NH2 and COOH groups are attached to different carbon atoms.
**Analysis:** This statement is incorrect. In alpha amino acids, the amino group (NH2) and the carboxyl group (COOH) are both attached to the same carbon atom, known as the alpha carbon. Therefore, this statement is false.

### Conclusion
The statement that is not correct is **Statement 4**: "In alpha amino acids, NH2 and COOH groups are attached to different carbon atoms."

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